1. Field of the Invention
This invention relates to an image forming apparatus for performing image formation using an electrophotographic technique, an inkjet technique or the like, and relates to a control method of the image forming apparatus.
2. Description of the Related Art
To ensure color stability of an output image from an image forming apparatus, various techniques for color stabilizing control have conventionally been proposed. These techniques are designed to read a pattern of a toner patch image formed, for example, on a surface of a photosensitive member in an electrophotographic printer using a density sensor from which the read information is fed back to a toner density controller of a developing unit, which carries out control to produce the appropriate toner density (see for example Japanese Laid-Open Patent Publication (Kokai) No. H01-309082).
The toner patch image is generally easily formed and removed. However, the toner density information can be obtained only before the toner image is fixed on a sheet, and therefore, the toner density control based on such toner density information cannot reflect influences occurring during and after the fixing process.
To obviate this, a method has been proposed in which an image is formed on an output sheet by, for example, a printer unit of a copying machine, the image is read by a reader unit attached to the printer unit, and image control is performed based on the result of read image (see Japanese Laid-Open Patent Publication (Kokai) No. S62-296669, for example). With this method, a user is required to perform complicated operations such as picking up an output sheet formed with an image from a sheet discharge unit of the copying machine, feeding the output sheet to the reader unit, and setting the reader unit to be ready for image reading.
To eliminate the complexity of operations, there has been disclosed a technique of detecting an output image formed on a sheet using a sensor disposed along a conveying path on the side downstream of a fixing device for fixing a toner image on the sheet (see Japanese Laid-Open Patent Publications (Kokai) No. H10-193689, for example).
Also in the field of inkjet printer, there is a problem that print color can vary due to variation in ink discharge amount with the passage of time, differences in environment, individual differences between ink cartridges, and the like. To eliminate this problem, there has been put on the market an inkjet printer that uses a density sensor disposed next to an ink head to accurately grasp and control the color stability of printed ink on a sheet.
Irrespective of whether the printer is based on the electrophotographic technique or the inkjet technique, the most important issue is to maintain the color stability. Also important is to commercialize products designed in consideration not only of technical improvements but also of users' operability. Thus, much attention is focused on the color stabilizing control of an output image using the sensor disposed along the conveying path on the side downstream of the fixing device.
However, the color stabilizing control on an output image using the sensor disposed downstream of the fixing device as in the prior art entails a problem that the color stabilizing control cannot be carried out with sufficient accuracy since the accuracy of detecting the density of a test pattern can be lowered due to variations in sensor characteristics and the like.
As the sensor disposed downstream of the fixing device, an optical color sensor is mainly used that includes a light emitting element, a light receiving element, and a color filter. The detection accuracy of this sensor can be lowered due to, for example, variations in characteristics of these components. For instance, when test patterns of the same density are detected at the same light source intensity, an output value for a yellow test-pattern image obtained through a blue filter is smaller than those of magenta and cyan test-pattern images respectively obtained through green and red filters. In other words, an S/N ratio is low in the detection of the yellow test-pattern image. Similar phenomenon can be caused due to variations in spectral characteristic of the light emitting/receiving element and can be also caused depending on the density of image pattern to be detected.
To improve the S/N ratio, a method for adjusting light emission intensity has been proposed. However, this method has drawbacks that the light intensity can vary due to heat generation caused when electric current is increased to increase the light intensity and that the output voltage from the light receiving element can saturate when a change occurs in conditions. Heretofore, an adequate countermeasure therefor has not been taken.